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nowinaminute

Anyone know if the old "2 slot" Mabuchi with the black end cap has advanced timing?

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I know it can be hard to tell with all these variations of 27T motors, especially sealed ones, but I have a vehicle with one of these motors and by ear at least, it seems a little faster in "reverse" on this particular vehicle, despite the exact same full pack voltage coming out of the motor wires whether in full reverse or full forward.

So I wondered if maybe the motor has advanced timing but because of the side of the vehicle it's mounted on with this particular one, combined with the gearing, it's effectively retarded because the motor has to run in reverse? 

I got the same voltage across the motor terminals whether in FWD or REV and with the motor out of the car.

Obviously it depends not only on the side of the gearbox the motor goes, but the gearing, too, but we know with stuff like the Clod rear gearbox that it's possible to have a situation where the motor has to run in reverse to make the wheels go forward.



 

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Other possibilities include the motor being worn out but I've never known that to be in one direction only. Also, I guess it could genuinely have restarded timing due to QC variations.

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They are nominally 0 timing as far as I know. Manufacturing variation is the most likely culprit, probably alignment of the brushes (there are no brush hoods in there, the brushes just hang in space at the end of little springy metal tabs). Not too hard to imagine the leading edge or trailing edge of one brush or the other sitting on the comm a little bit behind the 0 degree mark, which would have the effect of retarding the timing. The next one off the assembly line could have just as easily been advanced a tiny bit. These are industrial motors repurposed for RC; it was meant to move a printing platen up and down or pump coolant through pipes, and exact RPM and motor timing don't matter so much in those aplications.

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I recall I came across this in one of my previous lives.   It happens because the brushes wear on the leading edge, and the timing becomes retarded due to the wear pattern.   If the motor was brand new, the speed would be the same in both directions.

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This ⬆️

The old thing to do, was a 'reverse break in' to wear the brushes ,so as to give a few degrees of advanced timing, enough to add 200 - 300 extra rpm.

Other silver can tips.

2020-04-26_08-14-25

 

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Well, I took the motor out and tested it externally and even direct to the battery, there was an obvious difference in speed going in reverse.

Plenty of life left in the brushes and comm but impossible to tell if they have shifted a little one way or another. As markbt73 said, the brushes are just dangling of tabs in sealed can motors so there's a lot of potential for them to shift and perhaps settle in a slightly offset position.

I ended up just replacing the motor although interestingly, I had to go through a few unused stock motors (rubber thing still covering the shafts) till I found one with roughly the same speed in both directions. Not sure if the other ones were faster in forward or reverse as I was just going by ear, but it's interesting to note the Mabuchi datasheet for the RS-540RH and SH shows they have some timing applied. 

image.png.18aeff671ab5c62e041b62f0f1d084e1.png
 

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